1. Field of the Invention
The present invention relates generally to semiconductor processing and, more particularly, to chemical vapor deposition.
2. Related Art
Atomic layer deposition (ALD), also known as atomic layer chemical vapor deposition (ALCVD), is a method for producing very thin films that are highly conformal, smooth, and possess excellent physical properties. ALD uses volatile gases, solids, or vapors that are sequentially introduced (or pulsed) over a heated substrate. A first precursor is introduced as a gas, which is absorbed (or adsorbed) into the substrate and the reactor chamber is cleared of the gaseous precursor. A second precursor is introduced as a gas, which reacts with the absorbed precursor to form a monolayer of the desired material. By regulating this sequence, the films produced by ALD are deposited a monolayer at a time by repeatedly switching the sequential flow of two or more reactive gases over the substrate.
For example, FIGS. 1 through 4 illustrate a typical deposition sequence for producing a hypothetical film “AB” using ALD. FIG. 1 shows a drawing 100 illustrating the first step in the exemplary ALD deposition sequence that will deposit a layer of film on a substrate 102.
Drawing 100 shows the introduction of a precursor for element A (i.e., pulse A), designated ARx, into the chemical vapor deposition (CVD) chamber (where “R” represents an arbitrary functional group and “X” represents the number of functional groups associated with the precursor). The precursor is absorbed (or adsorbed) onto the surface of substrate 102. The first step shown in drawing 100 must be performed below the pyrolysis temperature of the precursor so that the precursor does not spontaneously decompose.
FIG. 2 shows a drawing 200 illustrating the second step in the exemplary ALD deposition sequence. Drawing 200 shows an inert purge gas that is introduced into the CVD chamber to remove the precursor gas ARX to prevent it from directly reacting with the following precursor for element B. Consequently, any possibility of gas phase reaction between the two precursors is eliminated.
FIG. 3 shows a drawing 300 illustrating the third step in the exemplary ALD deposition sequence. Drawing 300 shows the introduction of a precursor for element B (i.e., pulse B), designated BLy, into the CVD chamber (where “L” represents an arbitrary functional group and “y” represents the number of functional groups associated with the precursor).
The third step shown in drawing 300 can involve the actual introduction of element B into the film (as shown in FIG. 3) or it can simply involve the reduction of precursor ARx (e.g., leaving only element A). The two precursors, ARx and BLy, will begin to react on the surface of substrate 102 during step three, with the AB compound being formed and the ligand R being evolved as a volatile species.
FIG. 4 shows a drawing 400 illustrating the fourth step in the exemplary ALD deposition sequence. Drawing 400 shows an inert purge gas that is introduced into the CVD chamber to remove the precursor for B from the CVD chamber. The surface reaction between the two precursors is finished, leaving a complete layer of film on substrate 102. In some cases as mentioned above, the compound BLy serves only as a reducing agent, in which case, only a monolayer of element A remains on substrate 102.
ALD is performed in single wafer reactors with gas flows diverted to bypass when not in use and the four steps described above for FIGS. 1-4 performed sequentially. The type of gas is selected or switched by either opening valves in front of manual lines or orifices or through the use of a divert scheme which sends the gas that is currently not being used for deposition directly to the system vent. The typical ALD approach may be sufficient for research and development or low-volume production, but has a number of limitations.
For example, the throughput of the typical ALD approach is limited. The limitations of gas switching technology, as well as the time required to purge the single wafer showerhead and reactor, limit the total cycle to a typical time of approximately seven seconds. For a typical film having a single atomic layer of 2 Å, this translates to a deposition rate of approximately 17 Å per minute and, for a 40 Å layer, requires almost a three minute deposition time. This limits the overall throughput of the CVD module or reactor to approximately 20 wafers per hour, which is unacceptable for most production requirements. Single wafer processing chambers also have a significantly higher wafer transport and scheduling overhead as substrates must be moved to several modules.
Another drawback of single wafer systems is that in order to achieve the highest levels of throughput, the gas flow upstream of the throttle valve must be kept constant. Furthermore, to avoid recirculation and particle formation, it is desirable for the reactor chamber pressure to remain constant throughout the ALD cycle. Constant flows are necessary because the cycle times are too short for a typical throttle valve pressure control system to adequately respond. With a single wafer system, this requires a complex manifold in which the reactant gas flows are diverted downstream of the throttle valve and replaced by an equivalent purge flow.
Additional drawbacks of single wafer reactors include cost and repeatability issues. For example, the relatively complex manifold design inherent to single wafer systems leads to a high system cost. This is especially true because several single wafer chambers are required to provide a reasonable throughput. Additionally, each single wafer system consumes a position on a high-vacuum wafer transfer system. If the wafer transfer system is fully populated with single wafer systems to provide adequate throughput, a sufficient number of slots may not be available for other tasks. Also, single wafer systems operating in a parallel mode may have a bimodal distribution of process results, because all wafers are not experiencing the same process environment. This can lead to longer process startup and qualification times.
As a result, there is a need for improved systems and methods for the atomic layer deposition of thin films.